Copper Complexes in Carbon Nanotubes as Catalysts for Thermal Decomposition of Energetic Oxidizers

Metal compounds exhibit high catalytic activities in solid propellants as burning rate catalysts (BRCs), while the bulk particles and the nanoparticles loaded onto the surfaces of carbon materials cannot effectively display their catalytic activities. For reducing particle aggregation and improving their catalytic efficiencies as BRCs, seven copper complexes (CuL2) were successfully encapsulated into the inner spaces of carbon nanotubes (CNTs) via ultrasonication in this study. These complexes include Cu(Sal)2 (Sal = salicylate), CuC2O4, Cu(NO3)2·3H2O, Cu(acac)2 (acac = acetylacetonate), [Cu(TMEDA)2](NO3)2 (TMEDA = tetramethylethylenediamine), [Cu(MIM)4](DCA)2 (MIM = 1-methylimidazole, DCA = dicyanamide), and [Cu(NMIM)4](DCA)2 (NMIM = 1-methyl-2-nitroimidazole). In addition, the structures of the CuL2@CNT nanocomposites were investigated using transmission electron microscopy, scanning electron microscopy, Brunauer–Emmett–Teller surface area analysis, X-ray photoelectron spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray diffraction. Moreover, the combustion catalytic performances of the nanocomposites in the thermal decomposition of ammonium perchlorate (AP), cyclotrimethylenetrinitramine (also known as RDX), and 1,1-diamino-2,2-dinitroethene were evaluated; these performances considerably affect the thermal degradation of AP and RDX. The 5 wt % Cu(acac)2@CNTs with outer diameters of 4–6 nm (L1) caused the peak temperature of AP to shift 92.8 °C toward left at the high-temperature decomposition stage, and the released heat increased by 1448.06 J g–1 compared to pure AP; the 5 wt % [Cu(NMIM)2](NO3)2/@CNT (L1) advanced the RDX peak temperature by 17.3 °C. Moreover, the thermal decomposition mechanism of RDX in the presence of Cu(acac)2@CNT (L1) was investigated via in situ solid FTIR and thermogravimetry–FTIR–mass spectrometry. The additive (CuL2@CNTs) accelerated the exothermic reaction of C–N bond breakage. This in turn reduced the endothermic reaction of the N–N bond cleavage in RDX, contributing to an increase in the heat released by RDX. Based on these results, a potential mechanism is proposed where RDX pyrolysis is catalyzed by the composites